The present invention relates to liquid-level monitoring or measuring apparatus and, more particularly, to the transmitter unit for such apparatus.
Various apparatus are known for monitoring or measuring liquid level in a container or other confined space. One such apparatus employs a series-connected array of resistors whose inter-resistor junctions are tapped through individual magnetically actuatable glass enclosed reed switches to a common bus to form a series of .pi. networks. The reed switches are closed in sequence in response to tracking of the liquid level by a float borne permanent magnet. The electrical effect is akin to adjusting the slider on a resistance potentiometer whereby a suitable meter can respond to a signal for indicating liquid level or the like.
An example of such prior apparatus is described and claimed in U.S. Pat. No. 4,284,904 of Roland G. Tetro, issued Aug. 18, 1981, and assigned to the same assignee as the present application. As described in said patent, each network consisting of two resistors and one reed switch is separately encapsulated to provide a circuit element of unit length. Each encapsulated network has projecting terminal-strip elements by which the network is secured to a base strip or carrier of fiber-glass-filled plastic. The networks are connected via their projecting terminal-strip elements in tandem to form an array of any desired length. However, since the networks are connected end-to-end and are of unit length, the resolution sensitivity of this apparatus as limited to the dimension of the unit of length.
The array of reed switches such as that described in the aforementioned patent will be referred to herein as the "transmitter" of the apparatus, and the transmitter is normally inserted into a housing of some form to isolate it from the fluids being monitored. At the same time the housing provides a guide for the float element.
Another form of transmitter is known in which the resistors and glass enveloped reed switches are mounted directly on a strip of fiberglass reinforced plastic. In this latter version of the transmitter the reed switches are accurately mounted in a vertical echelon-like pattern such that as the magnet-carrying float rises or falls with the liquid level more than one switch is closed at any one instant. As will be explained later in the detailed description, this known arrangement is capable of providing increased resolution sensitivity. For example, if when the transmitter is vertically positioned the uniform inter-switch spacing is such as to position the reed switches at 1/2" intervals from the bottom to the top, and if the float as it moves closes switches in a 2-3-2 pattern, the transmitter will have a resolution sensitivity of 1/4".
The echelon type transmitter has been produced in a size that fits within a tubular housing having an inside diameter of about 3/8". Unfortunately, this type of construction has a length limitation determined by the maximum length of carrier substrate that can be obtained. At present, it appears that there is a 60" limit on the size available from the manufacturers of such substrate material.
It is, therefore, an object of the present invention to provide a replacement for the echelon type transmitter sized to fit within a 3/8" I.D. housing, which is free of any length restraint within practical limits.
It is a further object of the invention to provide such replacement in modular form and with the same liquid-level resolution capability as the previously known transmitter.